I'v had s_x through all of my pregnancys without any problems. Here's what The March of Dimes says:
http://www.marchofdimes.com/pnhec/159_516.asp
Unless your health care provider advises you otherwise, s_x during pregnancy is safe for you and your baby.
Many expectant parents worry that s_x can be harmful during pregnancy. They fear that intercourse could hurt the baby, or even cause miscarriage. Some are afraid that the baby somehow “knows” that s_x is taking place. The partner sometimes worries that intercourse might cause discomfort or pain for the pregnant woman. Worries like this are common and completely normal, but most of them are unfounded.
If your pregnancy is considered to be high risk, you may need to be more cautious than other women. Your health care provider may advise you to avoid intercourse for all or part of your pregnancy.